This invention relates generally to splicing two optical fibers together end-to-end. In particular this invention relates to apparatus and methods for splicing one segment of polarization maintaining (PM) fiber with an elliptical stress member to another segment of the same type of fiber.
Fiber optic splices are used in making fiber optic rotation sensors and other fiber optic devices. A fiber optic rotation sensor uses the Sagnac effect in a coil of optical fiber to detect rotations about a sensing axis that is perpendicular to the plane of the coil. Counterpropagating light waves in the sensing coil experience a phase shift that is related to the rotation rate. The phase shift is seen as a change in the interference pattern the waves make when they are combined. The interference pattern is produced when two waves of the same polarization have traversed the fiber optic sensing coil in opposite directions and then interfere. The interference pattern may be monitored by directing it onto a photodetector, which produces an electrical signal indicative of the intensity of the light in the interference fringe pattern. Therefore, there are advantages to using polarization-maintaining fiber in forming a fiber optic rotation sensor.
A typical fiber optic rotation sensor includes components such as phase modulators and polarizers formed on an integrated optics substrate. An optical signal is input to a pair of optical waveguides formed on the integrated optics substrate. The integrated optics substrate ordinarily has fiber optic leads connected to the optical waveguides. The fiber optic sensing coil also has leads that are butt-coupled to the fiber optic leads that extend from the optical waveguides. Difficulty is sometimes encountered in achieving proper alignment of the two polarization-maintaining fibers being spliced together.
The polarization-maintaining fiber has polarization-dependent refractive indices. The speed of light in an optical fiber is v=c/n, where n is the refractive index. Because the refractive index depends upon the polarization, the polarization having the larger refractive index will have a smaller propagation speed in the fiber than the polarization having the smaller refractive index. An optical fiber that has different refractive index for the two possible polarizations is said to be birefringent. The two polarizations are therefore sometimes called the "fast" wave and the "slow" wave. The birefringence of an optical fiber may be characterized by two principal axes of birefringence. The polarization of a linearly polarized light wave input to the fiber with the direction of polarization parallel to one of the principal axes of birefringence will be preserved, or maintained, as the wave propagates along the length of the fiber. The light wave in the fiber will thus be either fast wave or the slow wave. An optical signal having polarization components along both principal axes of birefringence will have a first portion coupled into the fast wave in the fiber and a second portion coupled into the slow wave.
If the polarization of a signal is to be maintained at a splice, the principal axes of birefringence of the two optical fibers must be aligned in parallel. Otherwise, part of the fast wave in one fiber couples into the slow wave of the other fiber, which is a phenomenon called polarization cross coupling. If the polarization is to be maintained at the splice, polarization cross coupling is undesirable.
In some fiber optic rotation sensor applications depolarized light is input to the sensing coil. Depolarized light is produced when linearly polarized light is incident upon an end of polarization-maintaining fiber with the axis of polarization at a 45.degree. angle to the principal axes of birefringence. In this arrangement, half of the incident light intensity couples into the fast wave and half into the slow wave.
Previously, the most common structure for polarization-maintaining fiber included a pair of rods in the cladding parallel to the core. The rods create a degree of birefringence in the such fibers such that the polarization of an input optical signal will be preserved as the optical signal propagates along the length of the fiber. Although this type of polarization-maintaining fiber structure provides generally satisfactory performance, the fiber has been found to be unduly expensive. Machines have been developed for splicing the earlier polarization-maintaining fibers together end-to-end. Such machines focus a light beam on the rods inside the fiber and align them before the fibers are spliced together with energy provided by an electric arc or other suitable energy source.
The newer fiber uses an elliptical stress member in the fiber core to produce the desired birefringence. This fiber has been found to have cost advantages over the previously used fiber. However, because of the internal structure of the new fiber, the standard passive techniques for aligning the birefringence axes are unusable. The new fiber does not include an internal structure that can be seen when the splicing machine focuses on the location where the rods could be seen in the older fiber.
One currently used splicing machine is the Fujikara model FSM-20 PM arc fusion splicer. This splicer normally aligns the birefringent axes of a pair of polarization maintaining fibers by utilizing a built-in imaging system. This system images the fibers from the side view and focuses on the stress inducing members that are used to create the birefringence feature of the fiber. When the machine images the new fiber from the side, no discernible features can be observed at the focal point of its built in imaging system.
Therefore, there is a need in the art for a technique for aligning these new fibers so that they may be spliced together end-to-end with the principal axes of birefringence of the two fibers being aligned.